The present invention relates to an `optomagnetic recording system` in which signals are recorded by irradiating a magnetic medium with optical energy to increase local temperature thereof and also applying a magnetic field thereto, and more particularly to an opto-magnetic recording system which can execute `overwrite` recording using a plurality of light beams.
Several techniques for executing overwrite recording have been proposed. One of them is to record information signals by irradiating an opto-magnetic medium with a light beam and also modulating a magnetic field.
This prior art technique is disclosed in JP-Appln. 63-20514 (which has been laid open as JP-A-196703) filed by the inventors of the present invention. In this technique, information signals are recorded on the magnetic medium of an opto-magnetic disk in such a manner that a signal at a single frequency is applied to a magnetic circuit to provide a modulator for modulating a light beam to be in synchronism with the signal, and a magnetic field generated by the magnetic circuit is synchronized with the information signal obtained by modulating the light beam. This technique will be explained in more detail with reference to FIGS. 7, 8 and 9.
FIG. 7 is a schematic view of a recording system according to the prior art (JP-A-1-196703). FIG. 8 is a timing chart of the operation in FIG. 7.
In FIG. 7, 201 is an opto-magnetic disk; 202 is a recording film having recording tracks (not shown); 203 is an objective lens; 204 is a light beam incident on the recording film, which constitutes a beam spot focused on the recording film by lens 203; and 205 is a magnetic circuit in which a coil 206 is wound. The magnetic circuit 205 generates a magnetic flux 207 when the coil 206 is supplied with a current, and a magnetic field H on the recording film 202.
The operation theory will be explained with reference to the timing charts of FIGS. 8A to 8E. FIG. 8A shows power of the laser light incident on the objective lens 203, which is modulated as shown. This laser light is focused as beam spots on the recording film 202 by the objective lens 203 as shown in FIG. 8E. The laser power has been modulated as shown in FIG. 8A so that the temperature of the recording film 202 varies as shown in FIG. 8B. When the magnetic field as shown in FIG. 8C is applied to the areas of the recording film 202 where the temperature exceeds T.sub.1, the temperature at each area exceeds the Curie point so that the magnetizing directions on these areas of the recording film 202 are defined as shown in FIG. 8D. The diameter of each of the beam spots as shown in FIG. 8E is set to be equal to or larger than the inverting interval of the magnetic field so that the magnetizing directions of S, N, S, . . . are alternatively recorded on the recording film 202 as shown in FIG. 8D.
FIG. 9 is a graph showing the temperature rise and fall in the recording film 202 when laser light is radiated to the recording film. The laser light used is pulsative light having a pulse width of 100 nsec. The recording film 202 formed on the opto-magnetic disk 201 moves at a relative speed of about 14.1 m/sec for the irradiating point of the laser light. The power of the irradiated laser light is set for about 3 mW. The laser power and the temperature of recording film 202 shown in FIGS. 8A and 8B, respectively, can be read in FIG. 9. In FIG. 8, the abscissa represents the distance (.mu.m) from the center of the radiated beam spot and the ordinate represents the temperature of the recording film. Distribution of the temperature after 100 nsec to 500 nsec has passed from the time when radiation of the laser pulse is started is illustrated. With the magnetic field always inverted in the directions of S and N, if the temperature of the recording film 202 is caused to exceed the Curie point when the magnetic field is in the S direction, the corresponding area is magnetized in the S direction. Likewise, the corresponding area can be magnetized in the N direction. In this way, information signals can be recorded on the recording film 202.
However, the prior art has the following drawback and problems remaining to be solved. Generally, in order to record information signals on and reproduce them from an opto-magnetic disk at a high speed, it is necessary to increase the relative speed between the opto-magnetic disk and an optical head. However, the relative speed is limited to a certain degree since the increased relative speed requires an increased laser output for recording. On the other hand, the present invention intends to record/reproduce information signals at a high speed without increasing the relative speed. In accordance with the present invention, a plurality of laser beams can be tracked on a plurality of tracks of the optical disk, and an overwrite function can be provided. If information signals are to be recorded or reproduced from different positions using plural light beams, the information signals recorded at the different positions must be in a fixed phase relation.
Further, the optical disk for recording information signals using the plural light beams must not be peculiar i.e., it must be a standard, commercially used disk. The present invention also intends to solve the above problems.